Apparatus for controlling the concentration and stability of an emulsion

ABSTRACT

A method of and system for controlling the oil concentration in and the stability of an oil-in-water emulsion used as a coolant-lubricant in a metalworking operation in which a portion of the emulsion is circulated through a sensor to monitor its density and to generate a signal relating the density to the oil concentration. The signal is employed to control addition of oil to the emulsion to thereby control the concentration within close limits. Sensing means also monitors the stability of the circulated emulsion, and emulsifier is added, as required, either to the oil to be added or directly to the emulsion to thereby control both the oil concentration in and the stability of the emulsion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to oil-in-water emulsions employed aslubricants in metalworking operations, and more particularly to a methodof and apparatus for monitoring the emulsion stability and oilconcentration and for controlling the addition of oil and emulsifiers tothereby control the concentration and stability of the emulsion.

2. Description of the Prior Art

It is well-known to use oil-in-water emulsions as lubricants inmetalworking operations. For example, in operation of rolling mills toshape metal such as aluminum and steel, it is conventional practice toemploy an oil-in-water emulsion to flood the work rolls and the metalbeing shaped as it enters the rolls. In such uses, the emulsion actsboth as a lubricant and as a coolant for both the work and the rolls.Such emulsions are also conventionally used in cutting, machining,milling, drawing, grinding and other like metalworking operations whereboth lubrication and temperature control may be importantconsiderations. The present invention was developed in connection withoperation of a steel rolling mill and reference to the operation of sucha mill will from time to time be made herein in describing theinvention. It is understood, however, that the invention is not limitedto such a system, but rather is equally applicable to systems for use inother metalworking operations.

A typical oil-in-water emulsion utilized in a steel rolling operationmay comprise an aqueous dispersion of a blend of oils, usually a mineraloil with natural oils such as tallow, along with nonionic and/or anionicemulsifying agents. The emulsion may also contain minor amounts of otheringredients such as bacteriocides and coupling agents which are employedto increase the useful life of the emulsion. A disclosure of suchemulsions may be found in numerous prior art publications, such, forexample, as U.S. Pat. Nos. 3,783,664 and 3,409,551.

In operation of a rolling mill, an oil-in-water emulsion isconventionally flowed or sprayed onto the work and/or rolls ahead ofeach set of work rolls. This necessarily results in a loss of a portionof the emulsion due to evaporation, drag-out, spillage, and the like.Variations in temperature of the metal, work rolls and ambient air canproduce variations in evaporation rate, particularly of the watercomponent of the emulsion, and various factors including surfaceconditions of the rolls and the metal being worked affect the loss ofwater, oil, emulsifiers, and other components. Thus, in order tomaintain a consistent oil concentration and emulsion stability, it isnecessary to periodically add varying amounts of oil, emulsifiers, andwater to the system.

The cooling and lubricating systems employed with rolling millsgenerally are either of the direct-application type, most frequentlyemployed with specialty orders and short runs, or of the recirculatingor reclaiming type. The reclaiming lubrication systems employed withsteel rolling mills typically include a large-volume storage chamber, ortank, which may incorporate means for controlling the temperature of theemulsion in the system. Emulsion pumped from this tank to the mill coolsand lubricates the workpiece and rolls, then flows downwardly and iscollected in a sump beneath the work rolls. The collected emulsion isthen purified through suitable filters, screens, and the like beforebeing returned to the storage tank for recirculation. Such systems mayhave a capacity of many thousands of gallons, and the emulsion in thesystem may be recirculated and used for several months of substantiallycontinuous operation. However, it is necessary to periodically analyzesamples of the emulsion to determine the oil concentration and emulsionstability, as well as other factors such as PH, bacteria count and thelike. Depending on the results of the sample analysis, make-up water,oil, emulsifiers and other components are added to control thestability, oil concentration, and other characteristics withinprescribed limits.

Direct application systems generally have a substantially lower totalemulsion capacity and do not incorporate the sophisticated reclaimingand purifying means employed in recirculating systems. Instead, re-useof the used emulsion is accomplished by pumping the used emulsion fromthe sump to a holding and mixing tank where it is continuously agitatedor mixed to prevent separation and from which it is pumped back to thepoint of application. Again, however, the composition and stability ofthe emulsion must be maintained within prescribed limits and this isaccomplished by the periodic addition of oil, water, emulsifiers, andthe like. As a general rule, the emulsion in a direct application systemis discarded after use for a comparitively short time such as after aspecified number of turns or after completion of a particular order orjob.

In the past, it has been conventional practice to periodically drawemulsion samples for laboratory-testing to determine oil concentrationand emulsion stability. The concentration has generally been determinedby centrifuging or by salt-split testing, both procedures beingwell-known in the art. The stability of the emulsion is represented byan emulsion stability index (ESI), which may be determined byconventional test procedures such as those contained in ASTM 3342-74,and which is an indication of the rate of separation of the oil andwater phases in a quiescent emulsion.

It has also been proposed to flow a portion of an emulsion through acontinuous centrifuge to separate the oil and water components which arethen caused to flow through separate flow meters which generate signalsproportional to the flow of the two components. A comparison of the twosignals is employed to periodically add slugs of oil, as required, tomaintain the proper concentration of oil in the emulsion. Such a systemis disclosed in U.S. Pat. No. 3,153,420.

U.S. Pat. No. 3,954,119 discloses a system for controlling the oilconcentration in an emulsion by measuring the rate of propagation ofultrasonic vibrations through the emulsion flowing in a pipe andrelating this measured rate to the emulsion density, and therby emulsionconcentration. The propagation rate is utilized to control the additionof oil and water to maintain the proper oil concentration in theemulsion.

SUMMARY OF THE INVENTION

According to the present invention, a rolling mill is provided with anoil-in-water emulsion lubricating system in which the oil concentrationmay be selected and automatically controlled at the selected value, witha continuous record being provided of the concentration of the oil inthe system. At the same time, the stability of the emulsion can beautomatically determined at preselected, frequent intervals, andemulsifiers added to the make-up oil or directly to the emulsion tomaintain the ESI within prescribed limits.

A portion of the emulsion is pumped through a density cell unit, forexample a Dynatrol liquid measuring density cell, which continuouslydetermines the density of the solution and generates a signalproportional to this density. The emulsion discharged from the densitycell is returned directly to the emulsion supply system, and the signalis fed to a continuous density recording and controlling system havingmeans for preselecting and setting the density, or oil concentration tobe maintained. A conventional, commercially available L&N recorderhaving an adjustable control set point may be employed to simultaneouslyrecord the oil density, on a continuous basis, and to supply a signal,through a conventional controller, to a motor control for controllingthe operation of an oil metering pump to supply make-up oil, as requiredto the system.

A Dynatrol liquid density cell employs a U-shaped tube through which theemulsion to be tested is pumped. The U-shaped tube is excited, and theamplitude of the displacement is carefully measured and employed as anindication of the density of the fluid contained in the tube. Dynatroldensity cells, per se, are commercially available and as such form nopart of the present invention.

In the conventional installation employing a Dynatrol density cell, theunit is mounted with the U-shaped tube in the horizontal direction.However, by mounting this unit with the tube in the vertical directionwith the open end of the "U" directed upward, in accordance with thepresent invention, the unit may also be employed to determine thestability of the emulsion. This is accomplished by periodically stoppingthe flow of emulsion through the unit and permitting the oil and waterin the U-tube to separate for a fixed length of time. At the conclusionof this fixed time, the density of the fluid in the U-tube isdetermined, and the change in density is employed to calculate theemulsion stability index by use of a mean time stability calculator. Thestability calculator generates a signal which may be employed toenergize a digital readout display and also to control the actuation ofan emulsifier pump to add a metered amount of emulsifiers to the oilwhich is supplied to the emulsion to maintain the concentration.Alternatively, the emulsifier may be added directly to the emulsion asin the emulsion mixing tank. The emulsion stability controller alsoemploys a set point control which may be adjusted to maintain thedesired ESI.

According to the present invention, the necessity for repeatedly drawingsamples of the emulsion for laboratory testing is eliminated. At thesame time, a continuous permanent recording of the emulsionconcentration is provided and the emulsion stability index is displayedon a visual readout in the oil room. Thus, not only is the concentrationand stability automatically maintained within very close limits, but thevisual display facilitates monitoring the system and a permanent recordis provided for subsequent correlation and evaluation of the millproduct.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the present inventionwill become more apparent from the detailed description containedherein, taken in conjunction with the drawings, in which:

FIG. 1 is a schematic illustration of a portion of a direct applicationlubrication system used to supply an oil-in-water emulsion to a rollingmill and incorporating means for controlling the oil concentration inthe emulsion;

FIG. 2 is a schematic illustration of a system similar to FIG. 1 andfurther including means for controlling the stability of the emulsion;

FIG. 3 is a typical recorder trace of the oil concentration as recordedon the apparatus of the present invention, with centrifuge test valuessuperimposed at periodic points for comparison purposes; and

FIG. 4 is a graphic comparison of oil concentration as determined by theapparatus according to the present invention and by centrifuge testingover a range of oil concentrations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, FIGS. 1 and 2 schematically illustrate acooling and lubricating system for supplying an oil-in-water emulsion tothe work rolls of a four-high rolling mill of the type employed, forexample, in cold rolling steel or aluminum sheet. The set of work rollsand back-up rolls are designated generally by the reference numeral 10,with a strip of metal 12 being illustrated as passing through the nip ofthe work rolls in the direction of the arrow 14. Upper and lowermanifolds 16, 18, respectively, are supported in position ahead of eachset of work rolls in the rolling mill to continuously flow or spray thecooling and lubricating emulsion onto the surface of the strip and workrolls as the strip enters each roll stand. The cooling and lubricatingemulsion (hereinafter emulsion) is supplied from an emulsion mixing tank20 by a pump 22 connected in pipe 24 to the manifolds 16, 18. Theportion of the emulsion which is not evaporated by the heat of the stripand work rolls or carried away by the strip, flows over the edge of thestrip and down into a mill sump 26 positioned below each roll stand 10in the mill. Used emulsion collected in the mill sump 26 is thenreturned to the mixing tank 20 through pipe 28 and suitable filtersindicated generally at 30.

The composition of the emulsion 32 contained in the mixing tank 20, andin the circulating system, may vary depending upon the nature of themetal working operation and normally will comprise an aqueous dispersionof a blend of oils and emulsifying agents to which small amounts ofother ingredients such as bacteriocides and coupling agents are added.The level of the emulsion is maintained in tank 20 by the addition ofwater from a suitable source, not shown, through a conduit 34 and ballfloat valve 36 and oil 38 from a holding tank 40 by a metering pump 42connected in pipe 44 extending from the holding tank 40 to the mixingtank 20.

Emulsifiers may be added to the oil in the holding tank or alternativelymay be added directly to the mixing tank, as required, to provide astable emulsion. An emulsifier holding tank 46 for containing a supply48 of an emulsifying agent or a suitable blend of emulsifying agents(hereinafter, emulsifier) is connected to the oil holding tank by a pipe50, and a metering pump 52 connected in pipe 50 is operable to delivermetered quantities of emulsifier to tank 40. Alternatively, theemulsifiers may be delivered directly from tank 46 to the mixing tank 20and mixed with the emulsion in the holding tank when required. Toaccomplish this, pump 52 may be connected in pipe 50 through a two-wayvalve 54 which may be selectively operable to direct the pump dischargeeither to tank 40 or tank 20. Operation of pump 52 may be manuallycontrolled, or alternatively it may be automatically controlled in themanner described hereinbelow with reference to FIG. 2.

The emulsion 32 in mixing tank 20 is continuously stirred and agitatedduring operation of the mill by a fan-like blade or propeller 56supported on a shaft 58 operatively connected to a motor 60 mountedabove the tank 20. The continuous agitation of the liquid maintains theoil dispersed throughout the water in very fine droplets, with theemulsifier acting to break up the oil in the conventional manner.

As previously described, a portion of the emulsion delivered throughmanifolds 16 and 18 to the metal working apparatus will inevitably belost so that it is necessary to periodically add quantities of thevarious components of the emulsion to make-up for the loss. However,since the various factors affecting loss of the emulsion may themselvesvary, the individual ingredients of the emulsion must be added invarying quantities from time to time in order to maintain the original,or desired composition of the emulsion. For example, an increase intemperature will produce an increased rate of loss of the water phase ofthe emulsion through evaporation whereas a variation in the compositionor surface characteristics of the metal being rolled can vary the rateof carryout of the oil phase. In accordance with the present invention,the oil concentration in the emulsion is continuously monitored andautomatically maintained at a predetermined level. This is accomplishedby pumping emulsion from the mixing tank 20 through an oil concentrationmeter or cell which continuously measures the density of the solutionand generates a signal proportional to the measured density. Theemulsion discharged from the cell is returned directly to the mixingtank, and in accordance with the invention, the signal from the cell isemployed to control the oil concentration in the emulsion withinspecified limits and to provide a continuous record of theconcentration.

As shown in FIGS. 1 and 2, a pump 62 is connected in supply pipe 64between the mixing tank 20 and a liquid density measuring cell 66 which,in turn, has its outlet connected to a return pipe 67 leading back totank 20. Thus, emulsion may be continuously circulated by the pump 62from the mixing tank through the concentration cell 66 and returneddirectly to the mixing tank. Oil concentration cell 66 is preferably acommercially available Dynatrol density cell, for example a Dynatroltype CL 10HY density cell employing a normally horizontally mountedstainless steel U-tube 68 through which the liquid whose density is tobe determined is flowed. In the embodiment of FIG. 1, the cell 66 ismounted in its normal position with the open ends of the U-shaped tubebeing rigidly supported and the closed loop end of the "U" beingunsupported. The closed end is excited at a fixed frequency, e.g.,120Hz., by a suitable coil 69 with the liquid flowing through the cell,and the amplitude of the tube displacement is accurately measured by apick-up 70 which generates an electric signal that is a function of themeasured tube displacement. The mass of the liquid flowing in the tubedirectly affects tube displacement so that measured displacement can bedirectly related to the liquid mass or density and consequently the oilconcentration in the emulsion.

The electrical signal generated by the density cell 66 is fed throughline 72 to a continuous recorder 71 calibrated to provide a continuous,permanent record of the oil concentration in the emulsion as determinedby the density cell. Recorder 71 may be an L&N Speedomax H strip chartrecorder, manufactured by Leeds and Northrop Corporation, having a chartspeed of two inches per hour and a response time of five seconds. Such arecorder is commercially available and has provision for adjusting thezero and span which may be set to accurately reflect the zero percentemulsion and a reading near the maximum, for example an emulsioncontaining 14% oil, as determined by centrifuging. The zero dial readingmay be set to zero when only the water employed in the emulsion isflowing through the density cell 66. The zero setting may be quicklychecked and adjusted as necessary to compensate for variations indensity of the water supply.

The recorder 71 also includes an adjustable control set point which maybe set to the desired oil concentration, and any deviation between theset point and the actual oil concentration as determined by the densitycell 66 will result in recorder 71 producing an electrical signalproducing an imbalance in a control circuit in a controller 73 which, inturn, directs a signal to motor control 74 controlling the operation ofpump 42. Controller 73 may be an L&N Series 80 controller, and motorcontrol 74 may be a Dynatrol motor control employed to control operationof the motor, for example, an eddy current AC motor, driving themetering pump 42 through a suitable gear reducer. Since such motors andgear reducers are conventional elements in driving metering pumps suchas the pump 42, they are not illustrated in the schematic drawings ofFIGS. 1 and 2.

In operation of the emulsion system shown in FIG. 1, when the signalfrom the Dynatrol density cell 66 indicates to recorder 71 that thedensity of the emulsion 32 in mixing tank 20 is below the required levelas set in the recorder, an imbalance signal is transmitted to thecontroller 73 which, in turn, transmits a drive signal to the motorcontrol 74 to drive the metering pump 42 at a rate to provide thedesired oil concentration within a relatively short time. The emulsionflowing from oil holding tank 40 flows directly into the mixing tank 20where the fresh oil is mixed by the continuously operating mixing blade56. As more oil is added to the emulsion 32, density cell 66 senses theincreased concentration and adjusts the recorder 71 to reflect theincrease. When the desired concentration is reached, the imbalancesignal to controller 73 is cancelled and pump 42 is stopped.

During operation of the system, as the level of emulsion 32 drops intank 20, fresh make-up water is automatically added directly to theemulsion tank through line 34 and ball float valve 36. The addition ofany substantial amount of make-up water is quickly sensed by the densitycontrol cell 66 as an increased density, or decreased oil concentration,and additional oil from holding tank 20 is added to bring theconcentration of the emulsion to the desired level in the manner justdescribed.

A predetermined quantity of emulsifier 48 from the holding tank 46 maybe initially added to the oil in holding tank 40 so that, upon eachaddition of oil, a proportionate amount of emulsifier is also added tomaintain the stability of the emulsion. Alternatively, the emulsifiermay be added directly to the emulsion within tank 32 when determined tobe necessary, as by taking samples of the emulsion in the mannerdescribed above. Two-way valve 54 may be selectively operated to directthe emulsifying agents either to holding tank 40 or to mixing tank 20,as desired.

Referring now to FIG. 2, a modification of the system is illustrated inwhich the stability of the emulsion is controlled by automaticdetermination of the ESI and automatic control of the addition ofemulsifier to maintain the stability of the emulsion within prescribedlimits. In this embodiment of the invention, the density cell 66 ismounted in the vertical direction, i.e., with the internal stainlesssteel U-tube 68 extending vertically so that emulsion flowing from pump62 enters the unit and flows upward through one leg, around the closedend, and downward through the other leg of the tube before exiting theunit to return through pipe 67 to the holding tank 20. A timingmechanism in recorder 71 is connected in the control circuit of pump 62through line 75, and periodically stops operation of the pump so thatflow of the emulsion through the density cell 66 is temporarilyinterrupted with the U-tube remaining filled with emulsion. A pair ofsedimentation amplifying emulsion reservoirs 76, 78 are attached one tothe bottom open end of each vertically extending leg of the U-tube 68with the reservoirs extending downwardly below the bottom end of theU-tube. The reservoirs 76, 78 are generally bottle-shaped and have adiameter substantially greater than the diameter of the U-tube 68.Reservoir 76 has its bottom end connected directly to supply pipe 64,and reservoir 78 has its bottom end connected to return pipe 67.

When the flow of emulsion through density cell 66 is stopped, the oiland water phases of the emulsion contained in reservoirs 76, 78 and inthe U-tube are permitted to separate, during which time the oil tends torise in the legs of the U-tube to the closed-loop end and the watertends to settle toward the open ends of the U-tube legs and into thereservoirs. By providing a reservoir of emulsion immediately beneath theopen ends of the two legs, the separation effect is amplified since oilseparating from the increased volume of the reservoirs will tend to morequickly displace water or emulsion from the smaller diameter U-tube.

During the time that flow through the density cell is stopped, thedensity is determined by exciting the U-tube in the normal manner. Theelectrical signal generated by the density cell 66, while the oil andwater are separating, is fed through line 80 to an emulsion stabilityindex meantime calculator 82. Calculator 82 compares the density signalsfrom cell 66 to the original density value, and employs this comparisonto calculate the mean time required for separation of the oil and waterphases of the emulsion. The mean time thus determined may then beemployed as an indication of the ESI of the emulsion.

An electrical signal representing the ESI determined by calculator 82 istransmitted to a controller 84 which is connected, through line 86, tothe pump 52 to control operation of the pump. Controller 84 is providedwith an adjustable set point which may be set to control operation ofthe pump 52 depending upon the output of the electrical signal from theESI calculator. Thus, when the calculator determines that the ESI isbelow the value set on controller 84, controller 84 then actuates thepump 52 to deliver a predetermined volume of emulsifier through valve 54to the emulsion in tank 20 or alternatively to the oil in holding tank40.

The signal from the calculator 82 is also led, through line 88, to adigital readout 90 which may be located at a remote position such as inthe oil control room to provide a readily visible digital readoutrepresentative of the stability index.

After a predetermined time sufficient for completion of the ESIdetermination, the timing mechanism in recorder 71 returns the system tothe normal operating mode by energizing pump 62 and conditioning densitycell 66 to feed the continuous electrical signal through line 72 torecorder 71. The timing mechanism in the recorder 71 then maintains thesystem in this condition to continuously monitor and provide a record ofthe emulsion density for a predetermined time before returning thesystem to the mode for checking the emulsion stability as described. Forexample, the recorder may be set to check the emulsion stability atintervals of eight to ten minutes in a normal metal rolling operation.

In an alternate arrangement, the signal produced by the ESI calculatorcan be employed only to energize the digital readout to signal the needfor the addition of emulsifier, with the addition of the emulsifierbeing manually controlled.

In a further alternative embodiment of the invention, two densitycontrol units may be employed, one connected in the system, as describedwith respect to FIG. 1, and a second density control unit employed toprovide the signal to the ESI calculator. In this arrangement, acontinuous record of the oil concentration in the emulsion is providedwith a second density cell being employed for the intermittentcalculation of the ESI to control the addition of emulsifier, andconsequently the stability of the emulsion.

In order to evaluate the invention under actual mill conditions, aDynatrol density cell of the type described above was installed in adirect application emulsion dispensing system having a 250 gallon mixingtank. The density cell was connected to an L&N strip chart recorderlocated on the mill floor at a location somewhat remote from the mill.The recorder was calibrated prior to installation in the mill, with thezero and span setting of the cell being determined experimentallyutilizing water and oil of the general type employed in the millemulsion.

FIGS. 3 and 4 represent typical data collected during the trial run ofthe system on the mill during use in the cold rolling of strip steel.FIG. 3 represents a typical recorder trace of the oil concentration asdetermined by the Dynatrol density cell and recorded on the L&Nrecorder. At various intervals during the operation, samples of theemulsion were taken from the mixing tank and the percent of oildetermined by centrifuging, using standard centrifuge procedure. Theresults of the centrifuge tests, and the emulsion temperature at thetime the samples were taken, are superimposed on the representation ofthe strip chart recording in FIG. 3.

FIG. 4 represents a correlation between the Dynatrol density cellreadings and the centrifuge test values for various oil concentrationsin an emulsion of the type employed in actual mill operations and thepercent of oil determined by centrifuging. The correlation in FIG. 4approaches a two-sigma accuracy of ±1.0%. The data was taken from adensity cell which had been operated for a period of approximately onemonth with only two adjustments to the zero setting after initialstart-up. The correlation between the Dynatrol cell readings and thecentrifuge testing was consistently better than the correlation betweencentrifuge testing and salt split testing conducted during the sameperiod. Thus, the tests clearly established that the apparatus accordingto the present invention may be readily employed to continuously monitorand control the oil concentration in an oil-in-water emulsion undercontinuous mill operating conditions.

Tests conducted under mill operating conditions employing the Dynatroldensity cell described above to determine the ESI have also shown thesystem to be reliable. By utilizing essentially the same equipment formonitoring both the oil concentration and emulsion stability, thestability index can be determined quickly and at frequent intervals toenable a more accurate control of the emulsion than was previouslypossible by the procedure of manually drawing a sample of the emulsionfor determination of the ESI. A more accurate control of the emulsionresults in a more uniform mill product and a substantial saving in bothoil and emulsifier.

While I have disclosed and described preferred embodiments of myinvention, I wish it understood that I do not intend to be restrictedsolely thereto, but rather that I do intend to include all embodimentsthereof which would be apparent to one skilled in the art and which comewithin the spirit and scope of my invention.

I claim:
 1. A lubricating system for supplying an oil-in-water emulsionlubricant to a metalworking operation comprising,a reservoir forcontaining a supply of emulsion, means for supplying fresh water to thereservoir, oil delivery means for supplying oil to the reservoir, meansfor mixing oil and water supplied to the reservoir to provide a uniformoil-in-water emulsion, means for supplying emulsion from the reservoirto a metalworking operation and for returning used emulsion from themetalworking operation to the reservoir, and emulsion stability controlmeans for controlling the stability of the emulsion in the system, thestability control means including, in combination, a liquid densitymeasuring cell operable to continuously measure the density of liquidtherein and to generate an electric signal which is a function of themeasured density, circulating means including conduit means connectingthe density measuring cell in the lubrication system for circulating aportion of the emulsion through the density measuring cell, controlmeans connected to said circulating means and operable to periodicallyinterrupt the circulation of emulsion through the density measuring cellwhile maintaining emulsion in the density measuring cell for apredetermined time sufficient to permit partial separation of the oiland water phases of the retained emulsion, the control means furtherincluding calculator means connected with and operable to compare thesignals from the density measuring cell at the beginning and end of thepredetermined time to determine the stability of the emulsion, and meansresponsive to the calculator means for dispensing an emulsifying agentfor addition to the emulsion to maintain the desired stability.
 2. Thelubricating system of claim 1 wherein said calculator means is connectedto said density measuring cell and is operable to determine the ESI ofthe emulsion, the system further comprising means for providing adigital readout of the determined ESI.
 3. The lubricating system ofclaim 1 wherein said calculator means is connected to said densitymeasuring cell and is operable to determine the ESI of the emulsion andincludes an adjustable set point for selecting the desired ESI, saidcalculator being operable to compare the determined ESI with the desiredESI, and wherein said means responsive to the calculator means isresponsive to a deviation between the determined ESI and the desired ESIto control the dispensing of an emulsifying agent.
 4. The lubricatingsystem of claim 1 wherein said density measuring cell comprises agenerally U-shaped steel tube mounted in a substantially verticaldirection with the open ends of the U-shaped tube being rigidlysupported and the closed loop end being unsupported, means for excitingthe unsupported end of the U-shaped tube at a substantially constantfrequency, and means measuring the displacement of the U-shaped tube. 5.The lubricating system of claim 1 wherein said control means furthercomprises recorder means for recording the electrical signal generatedby said density measuring cell.
 6. The lubricating system of claim 5wherein said recorder means comprises a strip chart recorder operable toproduce a continuous record of the electric signal, said strip chartrecorder being calibrated to record electric signals as the emulsiondensity in percent of oil in the emulsion.
 7. The lubricating system ofclaim 6 wherein said recorder includes an adjustable set point forselectively establishing a desired oil concentration, said recorderbeing operable to compare the measured oil concentration with theselected oil concentration and for controlling the addition of oil tothe emulsion when the deviation between the measured and selectedconcentrations exceed a predetermined minimum.
 8. The lubricating systemof claim 7 wherein said calculator means connected to said liquiddensity measuring cell is operable to determine the ESI of the emulsion,and further comprising means providing a digital readout of the desiredESI.
 9. The lubricating system of claim 8 wherein said calculator meansincludes an adjustable set point for selecting the desired ESI, saidcalculator being operable to compare the determined ESI with the desiredESI, said means responsive to the calculator means being responsive to adeviation between the determined ESI and the desired ESI to control theaddition of an emulsifying agent to the emulsion.
 10. The lubricatingsystem of claim 9 wherein said density measuring cell comprises aU-shaped steel tube mounted in a substantially vertical direction withthe open end of the U-shaped tube being rigidly supported and the closedloop end being unsupported, means for exciting the unsupported end ofthe U-shaped tube at a substantially constant frequency, and meansmeasuring the displacement of the U-shaped tube.
 11. A lubricatingsystem of claim 1 wherein said density measuring cell comprises anelongated steel tube shaped into the general configuration of aninverted "U" to define a pair of generally vertical, substantiallyparallel spaced legs joined at the top by a reverse bend portion of thetube,support means rigidly mounting the tube adjacent the bottom ends ofthe legs with the reverse bend portion and the top of the leg portionsof the tube being unsupported, means exciting the unsupported topportion of the tube at a substantially constant frequency, pick-up meansfor measuring the displacement of the tube, and emulsion reservoir meanssupported below and being connected to the U-shaped tube, the conduitmeans being connected to the emulsion reservoir means whereby emulsionflowing through the U-shaped tube also flows through the emulsionreservoir means.
 12. The lubricating system of claim 11 wherein theemulsion reservoir means comprises a pair of emulsion reservoirssupported below and being connected one to the bottom end of each of thelegs of the tube.
 13. The lubricating system of claim 12 wherein saidreservoirs are substantially cylindrical and have a diametersubstantially greater than the diameter of the U-shaped tube.